1. Field of the Invention
The present invention concerns a multi-layer ceramic capacitor and a manufacturing method thereof having a feature in an Ni internal electrode.
2. Description of the Related Technology
A multi-layer ceramic capacitor is manufactured by coating a conductive paste containing Ni metal particles by screen printing on a ceramic green sheet to form an internal electrode pattern, laminating a plurality of ceramic green sheets each formed with an internal electrode pattern and firing the obtained laminate.
The Ni conductive paste is fired simultaneously with the ceramic green sheet. Since the sintering initiation temperature for the Ni metal particles contained in the conductive paste is lower than the sintering initiation temperature for the ceramic layer formed of the ceramic green sheet, sintering of the Ni metal particles initiates first to initiate shrinkage. Subsequently, sintering of the dielectric ceramic layer initiates. In this case, the metal particles are fired further till the dielectric ceramic layer is sintered. Therefore, shrinkage proceeds further to result in a disconnected state such as a network-like or island-shape state in the internal electrode of the finished multi-layer ceramic capacitor. This decreases the area of the internal electrode to generate lowering of the electrostatic capacity.
The method of solving such disconnection of the internal electrode includes a method of adding a ceramic powder into a conductive paste to approach the temperature of firing shrinkage for the conductive paste to the temperature of firing shrinkage for the dielectric ceramic layer as disclosed in Japanese Unexamined Patent Publication Nos. Hei-8-078267 and 2001-122660. Further, a method of forming a cover layer of a noble metal having a higher melting point on the surface of an Ni metal particle of a conductive paste has been proposed as described in Japanese Unexamined Patent Publication No. 2004-319435.
However, the method of adding the ceramic powder in the conductive paste involves the following problem. As a result of grain growth of the metal particles in the course of firing, the ceramic powder and the metal particles are separated to discharge the ceramic powder into the dielectric ceramic layer. Generally, since the ceramic powder has poor wettability with the metal particles as the main ingredient of the conductive paste, the ceramic powder present between the metal particles is extruded by the metal particles during firing. The ceramic powders discharged in the course of the firing gather to form a columnar or network-shape to connect adjacent two dielectric ceramic layers. Further, since the grain growth of the metal particle is suppressed by the dielectric ceramic layers in the direction of the thickness, it proceeds so as to extend in the direction parallel with the plane direction of the internal electrode. Then, along with grain growth of the metal particles, since the conductive paste shrinks by sintering in the direction parallel with the plane direction of the internal electrode, a gap is formed between the metal particles along with shrinkage. The phenomena lower the continuity of the internal electrode to deteriorate the electrostatic capacity. Further, since the method of coating a noble metal at high melting point uses an expensive noble metal for coating the metal, the unit price of the metal particles is increased to diminish the merit of using Ni.
It is thus desirable to overcome the foregoing problems and provide a multi-layer ceramic capacitor having an internal electrode of good continuity at a relatively low cost.